1. Field of the Invention
This invention relates generally to vehicle braking devices, and more particularly to a vehicle braking device that operates to actuate and balance the braking of a towed vehicle.
2. Description of Related Art
Various brake control devices have been devised in the prior art for providing a towed vehicle with an independent braking system. The following prior art patents are illustrative the brake control devices that have been developed by the industry in the past.
Mullen, U.S. Pat. No. 2,856,036, discloses an automatic electric trailer brake control system that uses a hitch sensing device to automatically control electronic brakes. Whenever the trailer pushes against the towing vehicle, a microswitch triggers the electronic brakes in the trailer. An automatic rheostat functions to increase or decrease the strength of the braking, depending upon the length of time that the microswitch is closed. This type of control system sounds good in theory, but it does not work well in practice. Irregularities in the roadway and the natural swaying of both the towing vehicle and the trailer cause jolting of the hitch ball and unwanted and repeated activation of the trailer brakes. Not only does this create an uruiatural drag on the towing vehicle, this abuse also causes the trailer brakes to burn out. Furthermore, to minimize the impact of this problem, users of this system must set the brakes at a very low setting, causing the trailer brakes to be ineffective in emergency braking situations.
Since the pressures on the trailer hitch proved to be too unreliable to govern the brakes of a trailer or other towed vehicle, the art moved away from this solution and embraced instead various devices that sensed the change in momentum of the vehicle. These devices operate under the assumption that a change in momentum signifies rapid braking, and triggers the trailer brakes to help stop the vehicles. A first example of this approach, Stair, U.S. Pat. No. 3,053,348, discloses an electronically operated brake for a trailer that is actuated when the user engages the brakes of the towing vehicle. A pendulum pivots in response to either a change in momentum, such as a deceleration, or in response to the grade of the road, such as driving down a steep hill, and modifies the amount of braking force applied. A second example of this approach, Mizen et al., U.S. Pat. No. 3,990,749, discloses an inertia actuated servomotor for a trailer. The servomotor regulates a trailer's brakes in response to the movement of a weighted lever arm. The weighted lever arm is operably connected to the servomotor that controls the flow of vacuum from that towing vehicle, thereby actuating the brakes of the trailer.
This approach also failed under real-life driving conditions. The same imperfections in the road and irregularities in the trailer's momentum led to braking at undesirable times, which in turn made it difficult to calibrate braking strengths. The Stair device would trigger braking at merely a response to the grade of the road. The Mizen device would activate the brakes of the trailer when the user attempted to drive in reverse. While both devices functioned well during some driving conditions, neither device was able to function well during all driving conditions.
It is important to provide some braking of the towed vehicle to relieve the strain on the towing vehicle. It is especially desirable to provide strong braking in emergency-stop situations. However, it is undesirable to have the towed brake act as an "anchor" that helps pull the towing vehicle to a stop; and it is unacceptable for the brake control device to activate the towed brake when braking is not desired.
The prior art teaches various brake control devices that actuate the brakes of a towed vehicle in response to the actuation of the brakes of the towing vehicle. It is also known that it is desirable to modify this braking force in response to outside factors such as the rate of change of the momentum of the towed vehicle, and the grade of the road. However, the prior art does not teach a brake control device that only triggers the brakes of the towed vehicle when the brakes of the towing vehicle are actuated, and when the towed vehicle presses forward against the towing vehicle that is attempting to stop. The present invention fulfills these needs and provides further related advantages as described in the following summary.